Method for stimulating oil and gas reservoir volume by forming branch fractures in main fracture

ABSTRACT

The present invention provides a method for further forming a plurality of small fractures in a main fracture. If a radial main fracture is further fractured axially to form a plurality of branch fractures, the stimulated volume of the oil and gas reservoir will be achieved to greatly increase the yield of dense oil and gas. The main contribution of the present invention is to form three-dimensional fracture networks for any low-permeability oil reservoirs to achieve stimulated reservoir volume. The concept is that an original main fracture is further fractured to form a plurality of branch fractures (the branch fractures form included angles with the original main fracture), and the locations for forming the branch fractures in the main fracture and even the size and length of the branch fractures can be controlled artificially, so that the branch fractures can be formed by fracturing where needed.

TECHNICAL FIELD

The present invention relates to the technical field of oil and gas field development and provides a method for stimulating oil and gas reservoir volume by forming branch fractures in a main fracture.

BACKGROUND OF THE PRESENT INVENTION

Fracturing is the most important step in the development of oil and gas fields. Early fracturing is dominated by vertical well fracturing. With the development of horizontal drilling technology, horizontal well fracturing has become one of the most commonly used fracturing technologies. The staged horizontal well fracturing mainly includes mechanical staged-fracturing, dragging fracturing, pumping and fast drilling bridge plug fracturing and soluble bridge plug fracturing. These fracturing methods can form a main fracture at a perforation section of a horizontal well, resulting in the formation of a number of fractured fractures in the horizontal well to greatly improve the production yield of low-permeability oil and gas reservoirs. However, this method for forming a number of individual fractures in the horizontal well only achieves plane-cutting in the reservoir, and lower-permeability reservoirs, such as dense oil and gas reservoirs, require a stimulated reservoir volume to increase the yield of a single well and achieve efficient production of oil and gas wells.

At present, the major research direction of the universities and scientific research institutes is to continuously extend and elongate the main fractured fracture. With the research and development of pumping and fast drilling bridge plug, the original wellbore fracturing rules are broken, and tubing strings and other pipe strings in a wellbore are all lifted out, so that the original method is changed into a full-wellbore fracturing method. The fracturing effect of this full-wellbore fracturing is obviously improved at the initial stage, but many wells have become inefficient wells with their later development. Moreover, the original sand-carrying fracturing fluid causes some damage to the formation fluid. At the end of the 20th century, shale gas fracturing in the United States began to make extensive use of slickwater fracturing. The slickwater fracturing is mainly aimed at low-permeability and extra-low-permeability reservoirs, in order to communicate natural fractures so that the reservoirs form a complex fracture network structure to achieve the stimulated reservoir volume. The slickwater fracturing is mainly applied to the development of unconventional shale gas and dense oil and gas.

The large-displacement slickwater fracturing is a method to achieve the stimulated volume of oil and gas reservoirs at present. However, whether or not volume fracturing can be achieved in reservoirs depends on the reservoir properties which mainly are the brittleness of reservoir rocks, the development degree of natural microfractures and the ground stress difference. For shale reservoirs and dense oil and gas reservoirs with relatively low brittleness, undeveloped natural microfractures or large ground stress differences, it is also difficult to get the desired volume fracturing results in the reservoirs by slickwater fracturing.

SUMMARY OF THE PRESENT INVENTION

The present invention provides a concept for forming a branch fracture in a main fracture. If a radial main fracture is further fractured axially to form a branch fracture, the seepage of the oil and gas reservoir can be significantly improved. Thus, a concept for detonating explosives in a main fracture by pressurizing to form a branch fracture is provided.

A method for stimulating oil and gas reservoir volume by forming branch fractures in a main fracture is provided, comprising:

Method 1: forming a plurality of branch fractures in the main fracture in stages:

adding fracturing fluid to the main fracture in stages after the perforation is completed; adding fracturing fluid containing pressure-controlled explosives in the first stage, wherein the pressure-controlled explosives are located on the top of each main fracture after the completion of the fracturing fluid leak-off; injecting conventional fracturing fluid at high pressure and detonating explosives to form a plurality of branch fractures in the main fracture in the second stage; adding the conventional fracturing fluid in the second stage to increase the length of the main fracture and the branch fractures formed in the first stage; repeating the operations of the first and second stages in the third and fourth stages, during which the location, number and length of the branch fractures can be controlled with reference to the leak-off amount of the fracturing fluid into the formation and the amount of the pressure-controlled explosives.

Its basic principle is that the pressure-controlled explosives are placed into the main fracture in advance in stages, and then detonated to form branch fractures in the main fracture, which form certain included angles with the main fracture. During the whole process, the main fracture is lengthened while the branch fractures are formed in the main fracture, so that the main fracture forms a fracture network to improve the seepage of low-permeability oil and gas reservoirs and achieve the stimulated volume.

Forming a plurality of branch fractures in the main fracture in stages comprises:

a first step of fracturing the main fracture: running a pipe string for perforation and fracturing operations, and lifting the pipe string out after the completion of the operations;

a second step of injecting the fracturing fluid containing pressure-controlled explosives into the main fracture by a low-pressure pump; running a dual-packer single-stick pipe string which comprises a top packer, a pressure guide sleeve and a bottom packer, wherein the fracturing fluid containing pressure-controlled explosives is injected after the top packer and the bottom packer are set, and the pressure-controlled explosives are located on the top of the main fracture after the completion of the fracturing fluid leak-off;

a third step of injecting spacer fluid into the main fracture by a high-pressure pump and detonating the pressure-controlled explosives: injecting the spacer fluid into the main fracture by the high-pressure pump; and perforating the main fracture when the pressure in the main fracture reaches a detonating point of the pressure-controlled explosives and detonating the pressure-controlled explosives to form the branch fractures in the main fracture;

a fourth step of continuing injecting the fracturing fluid into the main fracture by the high-pressure pump to further increase the length of the main fracture; after the completion of perforation in the branch fracture, injecting the fracturing fluid by the high-pressure pump to increase the length of the branch fractures;

a fifth step of repeating the second step to the fourth step as needed: determining how many branch fractures need to be formed in the main fracture according to the requirements of fracturing design, and only repeating the second step to the fourth step; and

a sixth step of discharging the explosives from the main fracture and the branch fractures.

Method 2: forming a plurality of branch fractures in the main fracture at once:

after the completion of perforation, increasing the length of the main fracture by injecting fracturing fluid, then placing the explosives in the main fracture in advance, igniting the explosives in the main fracture by an igniter to form the branch fractures; injecting the fracturing fluid into the main fracture again after the initiation of detonation to increase the length of the branch fractures. During this process, the location and the length of the branch fractures are controlled according to the amount or concentration of the explosives injected.

The basic operation principle is to place the explosives into the main fracture, and then detonate the explosives by the electronic igniter to form one or more branch fractures in the main fracture, which form certain included angles with the main fracture, so that the main fracture forms a fracture network to improve the seepage of low-permeability oil and gas reservoirs and achieve the stimulated volume. Forming a plurality of branch fractures in the main fracture at once comprises:

a first step of fracturing the main fracture: running a pipe string for perforation and fracturing operations, and adding the fracturing fluid to increase the length of the main fracture after the completion of the operations;

a second step of adding fracturing fluid containing pressure-controlled explosives: in this step, a method for adding pressure-controlled explosives in stages is used, when there is fracturing fluid containing pressure-controlled explosives at different depths of the main fracture, alternately injecting the fracturing fluid and the fracturing fluid containing pressure-controlled explosives by pumps at time intervals by calculating leak-off amount and discharge amount, and placing the explosives at destination locations;

a third step of detonating the explosives by an electronic igniter: detonating explosives by an igniter after the explosives have been completely injected by the pumps, and re-perforating the main fracture to form the branch fractures; and a fourth step of fracturing the branch fractures: adding the fracturing fluid to the main fracture again, where, at this moment, the fracturing fluid permeates into the branch fractures to increase the length of the branch fractures, so that the main fracture and the branch fractures form a fracture network to improve the seepage of the oil and gas reservoir.

Beneficial Effects:

If a radial main fracture is further fractured axially to form a plurality of branch fractures, the stimulated volume of the oil and gas reservoir will be achieved to greatly increase the yield of dense oil and gas.

The main contribution of the present invention is to form three-dimensional fracture networks for any low-permeability oil reservoirs to achieve stimulated reservoir volume. The concept of the present invention is that an original main fracture is further fractured to form a plurality of branch fractures (the branch fractures form included angles with the original main fracture), and the locations for forming the branch fractures in the main fracture and even the size and length of the branch fractures can be controlled artificially, so that the branch fractures can be formed by fracturing where needed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1-1 is a side view of main and branch fractures in a wellbore of a vertical well according to the present invention;

FIG. 1-2 is a top view of the main and branch fractures in the wellbore of the vertical well according to the present invention;

FIG. 2 is a side view of main and branch fractures in a wellbore of a horizontal well according to the present invention;

FIG. 3-1 is a schematic diagram of a top end of the main fracture in method 1 according to the present invention;

FIG. 3-2 is a schematic diagram in which the branch fractures are formed at the top end of the main fracture in the method 1 according to the present invention;

FIG. 3-3 is a schematic diagram in which the main and branch fractures are lengthened in the method 1 according to the present invention;

FIG. 3-4 is a schematic diagram in which pressure-controlled explosives are added for a second time in the method 1 according to the present invention;

FIG. 3-5 is a schematic diagram in which the branch fractures are formed for a second time in the method 1 according to the present invention;

FIG. 3-6 is a schematic diagram in which the main fracture is lengthened and the branch fractures are formed for a second time in the method 1 according to the present invention;

FIG. 4-1 is a schematic diagram in which a long main fracture is formed at once in method 2 according to the present invention;

FIG. 4-2 is a schematic diagram in which electronically detonated explosives are injected based on process requirements in the method 2 according to the present invention; and

FIG. 4-3 is a schematic diagram of forming a plurality of branch fractures at once in the method 2 according to the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention will be discussed in detail below with reference to the accompanying drawings.

The present invention mainly aims at transforming a single fracture formed when fracturing the low-permeability reservoirs into a three-dimensional fracture network, so that the original main fracture is further fractured to form branch fractures which form included angles with the main fracture so as to achieve the stimulated volume. The locations for forming the branch fractures in the main fracture are controlled by the leak-off amount and the injection time of the fracturing fluid free of pressure-controlled explosives by pumps, and the length of the branch fractures is controlled by the injection time of the fracturing fluid, so that the branch fractures can be formed by fracturing where needed.

The stimulation of oil and gas reservoir volume by forming branch fractures in a main fracture is suitable for vertical wells, directional wells and horizontal wells, and the fracturing method thereof is pipe string dragging fracturing. Its key point lies in leak-off amount and concentration of carried explosives in the stage of injecting the pressure-controlled explosives.

The pressure guide sleeve is placed at a perforation section by running the pipe string, the fracturing fluid containing pressure-controlled explosives is injected into the main fracture, and part of the fracturing fluid will leak into the formation according to different geological environments and different kinds of fracturing fluid,

$C = \frac{2\mspace{14mu} \left( {1 - \gamma^{2}} \right)\mspace{14mu} H_{p}^{2}\beta}{3{EH}_{P}\sqrt{t}}$

where fp=H/Hp, dimensionless; Hp is the thickness (m) of a fractured layer; H is the fracture height (m); p is the fitting pressure (MPa); γ is the Poisson's ratio of the formation rock, dimensionless; E is the elastic modulus (MPa) of the formation rock; t is the injection time (s) of the fracturing operation; and β is the ratio of the average pressure in the fracture to the pressure in the well bottom, dimensionless. The leak-off amount of different kinds of fracturing fluid in different geological environments can be obtained by calculation.

The pressure-controlled explosives enter the main fracture along with the fracturing fluid and are finally placed on the top of the main fracture. Afterwards, spacer fluid is injected by a high-pressure pump to detonate the pressure-controlled explosives, and the fracture is perforated. Since the pressure-controlled explosives enter the main fracture along with the fracturing fluid, it only needs to determine the concentration of carried explosives or the like to control the density of the branch fractures. The flow conductivity

$F_{0} = {\frac{K_{1} \cdot W_{1}}{K \cdot L_{1}}\left( {1 - D} \right)}$

of the main fracture is calculated first to determine the concentration of carried explosives, and the concentration of carried explosives is calculated by a Nolte linear slope formula

$c = {c_{\max} \cdot \left( \frac{t - t_{0}}{t_{i} - t_{0}} \right)^{({1 - \sigma - {0.05\text{/}\sigma}})}}$

according to the reservoir parameters and the fracturing scale. 

What is claimed is:
 1. A method for stimulating oil and gas reservoir volume by forming branch fractures in a main fracture, wherein explosives are detonated in the main fracture by pressurizing to form a plurality of branch fractures, which comprises a step of forming a plurality of branch fractures in the main fracture in stages and/or forming a plurality of branch fractures in the main fracture at once so that the main fracture is changed into a fracture network to improve the seepage of low-permeability oil and gas reservoirs.
 2. The method for stimulating oil and gas reservoir volume by forming branch fractures in a main fracture according to claim 1, wherein forming a plurality of branch fractures in the main fracture in stages comprises steps of: adding fracturing fluid to the main fracture in stages after the completion of perforation; adding fracturing fluid containing pressure-controlled explosives in the first stage, wherein the pressure-controlled explosives are located on the top of the main fracture after the completion of the fracturing fluid leak-off, injecting conventional fracturing fluid at high pressure and detonating the explosives to form the branch fractures in the main fracture; adding the conventional fracturing fluid in the second stage to increase the length of the main fracture and the branch fractures formed in the first stage; in the third stage, repeating the operations of the first stage, and in the fourth stage, repeating the operations of the second stage, during which the location, number and length of the branch fractures are controlled according to the leak-off amount of the fracturing fluid into the formation and the amount of the pressure-controlled explosives.
 3. The method for stimulating oil and gas reservoir volume by forming branch fractures in a main fracture according to claim 2, wherein forming a plurality of branch fractures in the main fracture in stages comprises: a first step of fracturing the main fracture: running a pipe string for perforation and fracturing operations, and lifting the pipe string out after the completion of the operations; a second step of injecting the fracturing fluid containing pressure-controlled explosives into the main fracture by a low-pressure pump; and running a dual-packer single-stick pipe string which comprises a top packer, a pressure guide sleeve and a bottom packer, wherein the fracturing fluid containing pressure-controlled explosives is injected after the top packer and the bottom packer are set, and the pressure-controlled explosives are located on the top of the main fracture after the completion of the fracturing fluid leak-off; a third step of injecting spacer fluid into the main fracture by a high-pressure pump and detonating the pressure-controlled explosives: injecting the spacer fluid into the main fracture by a high-pressure pump; and perforating the main fracture when the pressure in the main fracture reaches a detonating point of the pressure-controlled explosives and detonating the pressure-controlled explosives to form the branch fractures in the main fracture; a fourth step of continuing injecting the fracturing fluid into the main fracture by the high-pressure pump to further increase the length of the main fracture; after the completion of perforation in the branch fracture, injecting the fracturing fluid by the high-pressure pump to increase the length of the branch fractures; a fifth step of repeating the second step to the fourth step as needed, and determining how many branch fractures need to be formed in the main fracture according to the requirements of fracturing design, and repeating the second step to the fourth step; and a sixth step of discharging the explosives from the main fracture and the branch fractures.
 4. The method for stimulating oil and gas reservoir volume by forming branch fractures in a main fracture according to claim 1, wherein forming a plurality of branch fractures in the main fracture at once comprises: after the completion of perforation, increasing the length of the main fracture by injecting fracturing fluid, then placing the explosives in the main fracture in advance, igniting the explosives in the main fracture by an igniter to form the branch fractures, injecting the fracturing fluid into the main fracture again after the initiation of detonation to increase the length of the branch fractures, and controlling the location and the length of the branch fractures according to the amount or concentration of the explosives injected.
 5. The method for stimulating oil and gas reservoir volume by forming branch fractures in a main fracture according to claim 4, wherein forming a plurality of branch fractures in the main fracture at once comprises: a first step of fracturing the main fracture: running a pipe string for perforation and fracturing operations, and adding the fracturing fluid to increase the length of the main fracture after the completion of the operations; a second step of adding fracturing fluid containing pressure-controlled explosives: in this step, a method for adding pressure-controlled explosives in stages is used, when there is fracturing fluid containing pressure-controlled explosives at different depths of the main fracture, alternately injecting the fracturing fluid and the fracturing fluid containing pressure-controlled explosives by pumps at time intervals by calculating leak-off amount and discharge amount, and placing the explosives at destination locations; a third step of detonating the explosives by an electronic igniter: detonating explosives by an igniter after the explosives have been completely injected by the pumps, and re-perforating the main fracture to form the branch fractures; and a fourth step of fracturing the branch fractures: adding the fracturing fluid to the main fracture again, where, at this moment, the fracturing fluid permeates into the branch fractures to increase the length of the branch fractures, so that the main fracture and the branch fractures form a fracture network to improve the seepage of the oil and gas reservoir. 